For many of those who wear watches, being able to read them in the dark is a major factor in deciding which watch to buy. The majority of watches made do not glow in the dark; of those that do, the length of time and brightness of the glow is dependent on the technology employed by the watchmaker. An alternate technology uses battery power to illuminate the watch dial when a button is pushed. However, since each use takes power from the battery, battery life is reduced.
Today's glow-in-the-dark watches use two basic means to achieve luminosity: a) the application of a phosphorescent material, such as Super-LumiNova® to the hands, dial and indices; and b) the use of tritium gas tubes. The use of phosphorescent material is the more popular method of providing luminescence, although there are pros and cons to both technologies.
Phosphorescent material works like a “light battery” that has to be “charged” before it outputs light energy. When you charge the phosphorescent material by light (sunlight or artificial light), the material's electrons are lifted to a higher quantum level. The stronger the activation light and the longer the exposure, the more electrons are lifted. In the dark, these lifted electrons return to previous energy levels, releasing energy in the form of light. Viewed in the dark, the luminosity of phosphorescent material is brightest at the beginning and then dies down until it eventually loses all its visible brightness.
In watches that use tritium (officially, gaseous tritium light source or GTLS), glass tubes holding tritium gas are placed within the watch. These tubes are made of borosilicate glass, which is temperature resistant. A coating of phosphorous material is applied to the inside of the tubes, which are then evacuated, filled with tritium gas and sealed. The level of brightness of these tubes is determined by the pressure of tritium in the tube, which is determined by the amount of tritium gas present (from 0 to 2.5 bar of gas). Although tritium has a half-life of twelve and a half years, a tritium gas tube is considered to have an operational luminosity between four to six years before its output level drops to below 50% of its original output level. When viewed in the dark, the luminosity of a tritium gas tube is lower than the initial luminosity of the phosphorous material, but remains constant over a larger period.
Each method of luminescence is selected based upon need. Tritium is used in areas where it's imperative that a timepiece be visible in total darkness, regardless of the availability of a light source. Phosphorescent material is used for all other applications, where luminescence is required only for a short period of time within a dark environment.